The present invention relates to a variable speed serial printing apparatus for a computer, word processor or the like.
A typical serial printer comprises a carriage which is fed relative to a sheet of paper and carries a rotary type or printing element such as a daisy wheel, ball or cylinder. The element is rotated to a position corresponding to an input data signal and then impacted against the paper through a ribbon for printing. The carriage is moved by one space each time a character is printed or an input data signal indicates an interword space or other non-printing operation.
The carriage may be driven in a stop-start (intermittent) manner or continuously. In the intermittent mode the carriage is stopped while printing each character. In the continuous mode the carriage is moved at a constant speed which is slow enough to allow sufficient time for selection of any character. Alternatively, the carriage may be driven at an average speed determined after sensing a plurality of input data.
A variation which has been recently introduced is to drive the carriage intermittently when the computed average speed is below a predetermined value and drive the carriage at the average speed when the average speed is above the predetermined value. Although this modification allows a slight increase in printing speed, the increase is not very great. The major problem with any carriage drive system involving intermittent drive is the large motor and power consumption required to start and stop the carriage in a very short length of time. The abrupt starts and stops create excessive stress on the mechanical parts of the printer and lead to early failure, even with frequent maintenance. The size of the motor and the power consumption may be reduced substantially in systems employing only continuous drive. However, a continuous drive system enabling increased printing speed in combination with decreased motor size and power consumption has not been developed heretofore.